Liposomal Camptothecins and Uses Thereof

ABSTRACT

This invention relates to improved liposomal camptothecin compositions and methods of manufacturing and using such compositions for treating neoplasia and for inhibiting angiogenesis.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Nos. 60/215,556, filed Jun. 30, 2000, and 60/264,616, filedJan. 26, 2001, both of which are hereby incorporated by reference intheir entireties for all purposes. U.S. patent application Ser. No.______, bearing Attorney Document No. 016303-008030, filed Jun. 29,2001, entitled “Liposomal Antineoplastic Drugs and Uses Thereof,” ishereby incorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

This invention relates to improved liposomal camptothecin compositionsand methods of manufacturing and using such compositions for treatingneoplasia and for inhibiting angiogenesis.

Therapeutic camptothecins, such as Topotecan(9-dimethylaminomethyl-10-hydroxy-camptothecin; Hycamtin™), andIrinotecan, are a semi-synthetic, water soluble derivative ofcamptothecin, an alkaloid extracted from the stem wood of theChinese'tree Camptotheca acuminata (Wall, et al., J. Am. Chem. Soc.88:3888-3890 (1966)). Camptothecins belong to the topoisomeraseinhibitor class of antineoplastic agents, specifically inhibiting theaction of the nuclear enzyme topoisomerase I which is involved in DNAreplication (Hsiang, at al., Cancer Res. 48:1722-1726 (1988)). As such,topotecan exhibits a cell cycle-specific mechanism of action, actingduring S-phase (DNA replication) to cause irreversible double strandbreaks in DNA that ultimately lead to G2 cell cycle arrest andapoptosis. In the free form, the drug has a broad spectrum of activityagainst a range of tumor cell lines and murine allograft and humanxenograft tumor models (McCabe, F. L. et al., Cancer Invest 12:308-313(1994); Emerson, et al., Cancer Res. 55:603-609 (1995); Thompson,Biochim. Biophys. Acta 1400:301-319 (1998); Ormrod, et al., Drugs58:533-551 (1999); Hardman, et al., Anticancer Res. 19:2269-2274(1999)). More recently, evidence has emerged that topotecan has stronganti-angiogenic properties that may contribute to its anti-tumormechanism of action (O'Leary, et al., Clin. Cancer Res. 5:181-187(1999); Clements, et al., Cancer Chemother. Pharmacol. 44:411-416(1999)). All these treatments are associated with dose-limiting toxicitysuch as non-cumulative myelosuppression leading to anaemia, neutropeniaand thrombocytopenia, and gastrointestinal-related toxicity, includingmucositis and diarrhea. Clinically, topotecan has been approved forsecond-line therapy in ovarian and small cell lung cancer (SCLC) and iscurrently the focus of extensive clinical evaluation.

Lipid formulations of camptothecins have been proposed as therapeuticagents (see, U.S. Pat. No. 5,552,156 and PCT Publication No. WO95/08986. However, not all lipid formulations are equal for drugdelivery purposes and extensive research continues into formulationswhich demonstrate preferred characteristics for drug loading andstorage, drug administration, pharmacokinetics, biodistribution, leakagerates, tumor accumulation, toxicity profile, and the like. Withcamptothecins, the field is further complicated because dose limitingtoxicities in humans may be 10-fold lower than in mice (Erickson-Miller,et al., Cancer Chemother. Pharmacol. 39:467-472 (1997)).

In short, camptothecins are a promising class of anti-neoplastic agents,and lipid formulations of these drugs could prove very useful. However,to date, lipid formulations have not provided sufficiently remarkableactivity to warrant clinical-advancement. It is an object of the instantinvention to provide novel lipid formulated camptothecins having novelclinical utility.

SUMMARY OF THE INVENTION

The present invention provides improved liposomal camptothecin (e.g.,topotecan) compositions having surprisingly increased clinical efficacyand decreased collateral toxicity. In addition, the present inventionprovides methods of using such liposomal camptothecin compositions totreat neoplasia and to inhibit angiogenesis.

In one aspect, the present invention provides a liposomal camptothecinunit dosage form comprising a camptothecin dosage of from about 0.015mg/M²/dose to about 1 mg/M²/dose and a lipid, wherein the liposomalcamptothecin unit dosage form has a drug:lipid ratio (by weight) ofabout 0.005 to about 0.01. In a preferred embodiment, the unit dosageform comprises a camptothecin dosage of from about 0.15 mg/M²/dose toabout 0.5 mg/M²/dose.

In one embodiment, the present invention provides a liposomal topotecanunit dosage form is about 0.01 mg/M²/dose to about mg/M²/dose and alipid and having a drug:lipid ratio (by weight) of about 0.05 to about0.2. In certain aspects, the drug:lipid ratio (by weight) is about 0.05to about 0.15. In another aspect, the liposomal topotecan unit dosageform is about 1 mg/M²/dose to about 4 mg/M²/dose of topotecan.

It will be readily apparent to those of skill in the art that any of thecamptothecins can be formulated in accordance with the presentinvention. In a preferred embodiment, the present invention providesliposomal topotecan unit dosage forms. In addition, it will be readilyapparent to those of skill in the art that any of a variety of lipidscan be used to form the liposomal compositions of the present invention.In a presently preferred embodiment, the lipid comprises a mixture ofsphingomyelin and cholesterol, preferably at a spingomyelin:cholesterolratio (by weight) of about 30:70 to about 60:40.

In another aspect, the present invention provides a liposomalcamptothecin (e.g., topotecan) formulation, wherein the formulationretains greater than 50% of the camptothecin as the active lactonespecies after 12 hours in blood circulation. In a preferred embodiment,the formulation retains greater than 80% of the camptothecin as theactive lactone species after 12 hours in blood circulation. In anotheraspect, the present invention provides a lipid formulation of acamptothecin (e.g., topotecan), comprising a camptothecin,sphingomyelin, cholesterol aid a divalent ionophore, such as a divalentcation ionophore. In a preferred embodiment, the divalent ionophore ispresent in trace amounts. In another preferred embodiment, the ionophoreis present in greater than trace amounts.

In still another aspect, the present invention provides a method oftreating a solid tumor in a human afflicted therewith, the methodcomprising administering to the human an effective amount of a liposomalcamptothecin unit dosage form in a pharmaceutically acceptable carrier.It will be readily apparent to those of skill in the art that any of avariety of solid tumors can be treated using the compositions of thepresent invention. In a preferred embodiment, the solid tumor to betreated is selected from the group consisting of solid tumors of thelung, mammary, colon and prostate. In another preferred embodiment, themethod further comprises co-administration of a treatment or activeagent suitable for treating neutropenia or platelet deficiency. In apreferred embodiment, the camptothecin dosage in the liposomalcamptothecin unit dosage form ranges from 0.015 mg/M²/dose to about 1mg/M²/dose. In another preferred embodiment, the liposomal camptothecinunit dosage form has a drug:lipid ratio (by weight) of about 0.005 toabout 0.01. In a preferred embodiment, the unit dosage form comprises acamptothecin dosage of from about 0.15 mg/M²/dose to about 0.5mg/M²/dose. Again, it will be readily apparent to those of skill in theart that any of the camptothecins can be formulated in accordance withthe present invention. In a preferred embodiment, a liposomal topotecanunit dosage form is used to treat the solid tumors. In addition, it willbe readily apparent to those of skill in the art that any of a varietyof lipids can be used to form the liposomal compositions of the presentinvention. In a presently preferred embodiment, the liposomal unitdosage form comprises a mixture of sphingomyelin and cholesterol,preferably at a spingomyelin:cholesterol ratio (by weight) of about30:70 to about 60:40.

Other features, objects and advantages of the invention and itspreferred embodiments will become apparent from the detailed descriptionwhich follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Pharmacokinetics and drug leakage of topotecan encapsulated inSM/CH vesicles. Plasma Topotecan (μg/ml) (A); Plasma Lipid (μg/ml) (B);and Relative Recovery (Topotecan/Lipid) (C). Topotecan was encapsulatedin 100 nm SM/CH (55/45 mol/mol) vesicles using the Mg-A23187 ionophoremethod as described in the Example section. Topotecan was assayed by afluorescence assay (see, the Example section) and lipid was determinedthrough the incorporation of a lipid marker, [³H]—CHE. Data pointsrepresent the mean±SD of 4 mice. The SM/CH formulation prepared by theMn-A23187 ionophore method is included in the bottom panel forcomparison (from Tardi et al., Cancer Res., 2000 submitted).

FIG. 2. Influence of mouse strain, dose schedule andliposome-encapsulation on topotecan tolerability in mice. Balb/c mice(CT-26); Single Dose (A); NCr nude mice (LX-1); Single Dose (B); NCrnude mice (MX-1); q3 d×4 (C); and NCr nude mice (MX-1); q7 d×3 (D). Bodyweights were monitored at least 3 times per week following drugadministration. Data points represent the mean (±SD) % change in bodyweight for each treatment group (n=6-8 mice). Data is from studiesNCTEF-002, NCTEF-003, NCTEF-006 and NCTEF-007.

FIG. 3. Efficacy of free (A) and encapsulated (B) topotecan in anintravenous L1210 murine leukemia model—single dose. L1210 cells (10⁵cells in 200 μl) were injected i.v. into the tail vein of BDF-1 mice onday 0. Topotecan was encapsulated in SM:CH (55:45) vesicles at adrug-to-lipid ratio of 0.10 (w/w) using the Mg-A23187 method asdescribed in Experimental. All dilutions were made in sterile 0.9%saline immediately prior to injection. Drug administration was made viathe lateral tail vein at 1 day post-tumor cell injection. Each groupconsisted of 8 animals. Data is from NCTEF-005.

FIG. 4. Efficacy of free (A) and encapsulated (B) topotecan in anintrasplenic CT-26 murine colon metastases model—single dose. CT-26cells (10⁴ cells in 50 μl) were injected into the spleen of Balb/c miceon day 0. Topotecan was encapsulated in SM:CH (55:45) vesicles at adrug-to-lipid ratio of 0.10 (w/w) using the Mg-A23187 method asdescribed in Experimental. All dilutions were made in sterile 0.9%saline immediately prior to injection. Drug administration was made viathe lateral tail vein at 1 day post-tumor cell injection. Each groupconsisted of 8 animals. Data is from NCTEF-002.

FIG. 5. Efficacy of free and encapsulated topotecan in subcutaneous MX-1human breast xenografts—single dose. Topotecan was encapsulated in SM:CH(55:45) vesicles at a drug-to-lipid ratio of 0.10 (w/w) using theMg-A23187 method as described in Experimental. All dilutions were madein sterile 0.9% saline immediately prior to injection. Drugadministration was made via the lateral tail vein at 11 days post-tumorcell implantation when the tumors were 100-300 mm³. Data pointsrepresent mean±SEM (n=6). Data is from NCTEF-004.

FIG. 6. Efficacy of free and encapsulated topotecan in subcutaneous LX-1human SCLC xenografts—single dose. Topotecan was encapsulated in SM:CH(55:45) vesicles at a drug-to-lipid ratio of 0.10 (w/w) using theMg-A23187 method as described in Experimental. All dilutions were madein sterile 0.9% saline immediately prior to injection. Drugadministration was made via the lateral tail vein at 10 days post-tumorcell implantation when the tumors were 100-300 mm³. Data pointsrepresent mean±SEM (n=6). Data is from NCTEF-003.

FIG. 7. Efficacy of free and encapsulated topotecan in subcutaneous MX-1xenografts q3 d×4 schedule. Topotecan was encapsulated in SM:CH (55:45)vesicles at a drug-to-lipid ratio of 0.10 (w/w) using the Mg-A23187method as described in Experimental. All dilutions were made in sterile0.9% saline immediately prior to injection. Drug administration was madevia the lateral tail vein beginning on day 10 (dosing days are indicatedwith an asterisk) post-tumor cell implantation. The dose (in mg/kg/dose)is indicated in parentheses in the figure. Data points representmean±SEM (n=6). Data is from NCTEF-006.

FIG. 8. Efficacy of free and encapsulated topotecan in subcutaneous MX-1xenografts—q7 d×3 schedule. Topotecan was encapsulated in SM:CH (55:45)vesicles at a drug-to-lipid ratio of 0.10 (w/w) using the Mg-A23187method as described in Experimental. All dilutions were made in sterile0.9% saline immediately prior to injection. Drug administration was madevia the lateral tail vein on day 12, 17 and 24 post-tumor cellimplantation. The dose (in mg/kg/dose) is indicated in parentheses inthe figure. Data points represent mean±SEM (n=6). Data is fromNCTEF-009.

FIG. 9. Efficacy of free and encapsulated topotecan in subcutaneous LX-1xenografts—q7 d×3 schedule. Topotecan was encapsulated in SM:CH (55:45)vesicles at a drug-to-lipid ratio of 0.10 (w/w) using the Mg-A23187method as described in Experimental. All dilutions were made in sterile0.9% saline immediately prior to injection. Drug administration was madevia the lateral tail vein on day 14, 21 and 28 post-tumor cellimplantation. The dose (in mg/kg/dose) is indicated in the figure. Datapoints represent mean±SEM (n=6). Data is from NCTEF-007.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

It has now been discovered that the anti-tumor activity of topotecanhydrochloride (Hycamtin™, SmithKline Beecham) encapsulated insphingomyelin/cholesterol Liposomes, such as sphingomyelin/cholesterol(55:45) liposomes, by a gradient loading method provides surprisingclinical efficacy at lower doses, and with lower collateral toxicity,than free topotecan. The data demonstrates a dramatic increase intherapeutic index of liposome-encapsulated topotecan relative to thefree drug. The present invention also provides a novel range ofdifferent dosages and dosage schedules, and a variety of drug:lipidratio formulations of liposomal camptothecins, that are useful fortreating solid tumors.

I. Compositions and Methods of Making Liposomal Camptothecins

Liposome, vesicle and liposome vesicle will be understood to indicatestructures having lipid-containing membranes enclosing an aqueousinterior. The structures can have one or more lipid membranes unlessotherwise indicated, although generally the liposomes will have only onemembrane. Such single-layered liposomes are referred to herein as“unilamellar.” Multilayer liposomes are referred to herein as“multilamellar.”

The liposomes that are used in the present invention are preferablyformed from lipids which when combined form relatively stable vesicles.An enormous variety of lipids are known in the art which can be used togenerate such liposomes. Preferred lipids include, but are not limitedto, neutral and negatively charged phospholipids or sphingolipids andsterols, such as cholesterol. The selection of lipids is generallyguided by consideration of, e.g., liposome size and stability of theliposomes in the bloodstream.

Preferred liposome compositions for use in the present invention includethose comprising sphingomyelin and cholesterol. The ratio ofsphingomyelin to cholesterol in the liposome composition can vary, butgenerally is in the range of from about 75/25 mol %/mol %sphingomyelin/cholesterol to about 30/50 mol %/mol %sphingomyelin/cholesterol, more preferably about 70/30 mol %/mol %sphingomyelin/cholesterol to about 40/45 mol %/mol %sphingomyelin/cholesterol, and even more preferably about 55/45 mol%/mol % sphingomyelin/cholesterol. Other lipids can be included in theliposome compositions of the present invention as may be necessary, suchas to prevent lipid oxidation or to attach ligands onto the liposomesurface. Generally, if lipids are included, the other inclusion of suchlipids will result in a decrease in the sphingomyelin/cholesterol ratio.Liposomes of this type are known as sphingosomes and are more fullydescribed in U.S. Pat. No. 5,814,335, the teachings of which areincorporated herein by reference.

A variety of methods are available for preparing liposomes as describedin, e.g., Szoka, et al., Ann. Rev. Biophys. Bioeng. 9:467 (1980); U.S.Pat. Nos. 4,235,871; 4,501,728; 4,837,028, the text Liposomes, Marc J.Ostro, ed., Marcel Dekker, Inc., New York, 1983, Chapter 1; and Hope, etal., Chem. Phys. Lip, 40:89 (1986), all of which are incorporated hereinby reference. The protocol for generating liposomes generally includes:mixing of lipid components in an organic solvent; drying andreconstituting liposomes in aqueous solvent; and sizing of liposomes(such as by extrusion), all of which are well known in the art.

Alternative methods of preparing liposomes are also available. Forinstance, a method involving detergent dialysis based self-assembly oflipid particles is disclosed and claimed in U.S. Pat. No. 5,976,567issued to Wheeler, et al., which avoids the time-consuming and difficultto-scale drying and reconstitution steps. Further methods of preparingliposomes using continuous flow hydration are under development and canoften provide the most effective large scale manufacturing process.

Preparation of liposomal camptothecins requires loading of the drug intothe liposomes. Loading can be either passive or active. Passive loadinggenerally requires addition of the drug to the buffer at the time of thereconstitution step. This allows the drug to be trapped within theliposome interior, where it will remain if it is not lipid soluble, andif the vesicle remains intact (such methods are employed, for example,in PCT Publication No. WO 95/08986, the teachings of which areincorporated herein by reference).

Active loading is in many ways preferable, and a wide variety oftherapeutic agents can be loaded into Liposomes with encapsulationefficiencies approaching 100% by using a transmembrane pH or iongradient (see, Mayer, et al., Biochim. Biophys. Acta 1025:143-151 (1990)and Madden, et al., Chem. Phys. Lipids 53:37-46 (1990)). Numerous waysof active loading are known to those of skill in the art. All suchmethods involve the establishment of some form of gradient that drawslipophilic compounds into the interior of liposomes where they canreside for as long as the gradient is maintained. Very high quantitiesof the desired drug can be obtained in the interior, so much that thedrug may precipitate out on the interior and generate a continuinguptake gradient.

Particularly preferred for use with the instant invention is ionophoremediated loading as disclosed and claimed in U.S. Pat. No. 5,837,282,the teachings of which are incorporated by reference herein. Basically,this method employs an ionophore in the liposome membrane to drive thegeneration of a pH gradient from a previously existing monovalent ordivalent ion gradient.

An important characteristic of liposomal camptothecins forpharmaceutical purposes is the drug to lipid ratio of the finalformulation. Drug:lipid ratios can be established in two ways: 1) usinghomogenous liposomes each containing the same drug:lipid ratio; or 2) bymixing empty liposomes with liposomes having a high drug:lipid ratio toprovide a suitable average drug:lipid ratio. For different applications,different drug: lipid ratios may be desired. Techniques for generatingspecific drug:lipid ratios are well known in the art. Drug:lipid ratioscan be measured on a weight to weight basis, a mole to mole basis or anyother designated basis. Preferred drug:lipid ratios range from about0.005 drug:lipid (by weight) to about 0.2 drug:lipid (by weight) and,more preferably, from about 0.01 drug:lipid (by weight) to about 0.02drug:lipid (by weight).

A further important characteristic is the size of the liposomeparticles. For use in the present inventions, liposomes having a size offrom about 0.05 microns to about 0.15 microns are preferred.

The present invention also provides liposomal camptothecin compositionsin kit form. The kit can comprise a ready-made formulation, or aformulation which requires mixing of the medicament beforeadministration. The kit will typically comprise a container that iscompartmentalized for holding the various elements of the kit. The kitwill contain the liposomal compositions of the present invention or thecomponents thereof, possibly in dehydrated form, with instructions fortheir rehydration and administration

The liposome compositions prepared, for example, by the methodsdescribed herein can be administered either alone or in a mixture with aphysiologically-acceptable carrier (such as physiological saline orphosphate buffer) selected in accordance with the route ofadministration and standard pharmaceutical practice. Generally, normalsaline will be employed as the pharmaceutically acceptable carrier.Other suitable carriers include, e.g., water, buffered water, 0.4%saline, 0.3% glycine, and the like, including glycoproteins for enhancedstability, such as albumin, lipoprotein, globulin, etc. Thesecompositions may be sterilized by conventional, well known sterilizationtechniques. The resulting aqueous solutions may be packaged for use orfiltered under aseptic conditions and lyophilized, the lyophilizedpreparation being combined with a sterile aqueous solution prior toadministration. The compositions may also contain pharmaceuticallyacceptable auxiliary substances as required to approximate physiologicalconditions, such as pH adjusting and buffering agents, tonicityadjusting agents and the like, for example, sodium acetate, sodiumlactate, sodium chloride, potassium chloride, calcium chloride, etc.Additionally, the composition may include lipid-protective agents whichprotect lipids against free-radical and lipid-peroxidative damages onstorage. Lipophilic free-radical quenchers, such as α.-tocopherol andwater-soluble iron-specific chelators, such as ferrioxamine, aresuitable.

Exemplary methods of making specific formulations of liposomalcamptothecins and, in particular, liposomal topotecan are set out in theexamples below.

II. Methods of Using Liposomal Camptothecins

Liposomal camptothecins are used, according to this invention, in thetreatment of solid tumors in an animal, such as a human. The examplesbelow set out key parameters of the drug:lipid ratios, dosages ofcamptothecin and lipid to be administered, and preferred dose schedulingto treat different tumor types.

Preferably, the pharmaceutical compositions are administeredparenterally, i.e., intraarticularly, intravenously, intraperitoneally,subcutaneously or intramuscularly. More preferably, the pharmaceuticalcompositions are administered by intravenous drip or intraperitoneallyby a bolus injection.

The concentration of liposomes in the pharmaceutical formulations canvary widely, i.e., from less than about 0.05%, usually at or at leastabout 2-5% to as much as 10 to 30% by weight and will be selectedprimarily by fluid volumes, viscosities, etc., in accordance with theparticular mode of administration selected. For example, theconcentration can be increased to lower the fluid load associated withtreatment. Alternatively, liposomes composed of irritating lipids can bediluted to low concentrations to lessen inflammation at the site ofadministration. The amount of liposomes administered will depend uponthe particular camptothecin used, the disease state being treated andthe judgement of the clinician, but will generally, in a human, bebetween about 0.01 and about 50 mg per kilogram of body weight,preferably between about 5 and about 40 mg/kg of body weight. Higherlipid doses are suitable for mice, for example, 50-120 mg/kg.

Dosage for the camptothecin will depend on the administratingphysician's opinion based on age, weight, and condition of the patient,and the treatment schedule. A recommended dose for free topotecan inSmall Cell Lung Cancer is 1.5 mg/M² per dose, every day for 5 days,repeated evert three weeks. Because of the improvements in treatment nowdemonstrated in the examples, below, doses of topotecan in liposomaltopotecan in humans will be effective at ranges as low as from 0.015mg/M²/dose and will still be tolerable at doses as high as 15 to 75mg/M²/dose, depending on dose scheduling. Doses may be single doses orthey may be administered repeatedly every 4 h, 6 h, or 12 h or every 1d, 2 d, 3 d, 4 d, 5 d, 6 d, 7 d, 8 d, 9 d, 10 d or combination thereof.Preferred scheduling may employ a cycle of treatment that is repeatedevery week, 2 weeks, three weeks, four weeks, five weeks or six weeks orcombination thereof. In one preferred embodiment, treatment is givenonce a week, with the dose typically being less than 1.5 mg/M². Inanother embodiment, the interval regime is at least once a week. Inanother embodiment, interval regime is at least once every two week, oralternatively, at least once every three weeks.

Particularly preferred topotecan dosages and scheduling are as follows:

Dosage (mg/M²/dose) Period Repeat Cycle every: 0.15 1 d × 5 d 3 weeks0.5 1 d 1 week 1.5 1 d 1 week 15 1 d 3 weeks 50 1 d 3 weeks

The invention will be described in greater detail by way of specificexamples. The following examples are offered for illustrative purposes,and are not intended to limit the invention in any manner. Those ofskill in the art will readily recognize a variety of non-criticalparameters which can be changed or modified to yield essentially thesame results.

III. Examples

A. Materials and Methods

1. Materials.

Topotecan (Hycamtin™, SmithKline Beecham) was purchased from thepharmacy at the British Columbia Cancer Agency. Sphingomyelin (SM) waspurchased from Avanti Polar Lipids. Sphingomyelin from Northern Lipidswas used in an early study, but was less soluble in ethanol than theAvanti version. Cholesterol (CH) and the divalent cation ionophoreA23187 were purchased from Sigma. [³H]-cholesterylhexadecylether(Dupont) was used as a lipid marker.

2. Mice.

Female, ICR, BDF-1 or athymic nu/nu (6-8 weeks) were purchased fromHarlan-Sprague Dawley (Indianapolis, Ind.). All animals were quarantinedfor one week prior to use. All studies were conducted in accordance withthe guidelines established by the Canadian Council on Animal Care (CCAC)and the Institutional Animal Care and User Committee (IACUC).

3. Formulation of Topotecan by the Mg-A23187 Method.

Topotecan was encapsulated in SM:CH (55:45, mol/mol) liposomes using theMg-A23187 ionophore method according to U.S. Pat. No. 5,837,282. Theinitial drug-to-lipid ratio was 0.10 (w/w) and drug loading wastypically 95-100%. The external buffer consisted of 10 mM PBS, pH 7.5and 300 mM sucrose. All formulations were analyzed with respect toparticle size, drug loading efficiency, pH, and drug and lipidconcentration.

4. Drug Preparation and Dosing.

Each vial of topotecan (Hycamtin™) was hydrated in 1.0 ml of sterilewater, giving a topotecan concentration of 4.0 mg/ml. Subsequentdilutions were μl made in 0.9% sterile saline to maintain the low pHrequired for the lactone species of the drug. Unused drug in the waterstock solution (4.0 mg/ml) was stored at 4° C. in the absence of light.Liposome encapsulated topotecan was diluted in 0.9% saline to therequired concentration for administration. All drug administrations wereat 10 ml/kg (200 μl/20 g mouse) via the lateral tail vein.

5. Pharmacokinetic and In Vivo Leakage Studies.

The pharmacokinetics and drug leakage of free and liposome encapsulatedtopotecan were evaluated in ICR mice over 24 h following i.v.administration via the lateral tail vein. Two different drug-to-lipidratios, i.e., 0.10 (w/w) and 0.02 (w/w), were used to examine theinfluence of drug-to-lipid ratio and lipid dose on drug leakage and PKbehavior. Encapsulated topotecan was administered at 1 mg/kg (10 or 50mg/kg lipid) and 5 mg/kg topotecan (50 mg/kg lipid). Correspondingly,the PK behavior of free topotecan was evaluated at and 1 and 5 mg/kg.Total topotecan in blood was determined by a fluorescence assay precededby precipitation of plasma proteins. Topotecan was quantified byspectrofluorimetry at an excitation (2.5 nm slit width) and emissionwavelength (2.5 nm slit width) of 380 and 518 nm, respectively. Lipidlevels in plasma were determined by liquid scintillation counting of the[³H]-CHE label.

6. MTD Studies.

MTD studies were performed in the host mouse strain corresponding toeach tumor model. Single dose and multidose MTD were determined bymonitoring weight loss over time. The MTD was defined as the dose thatresulted in 20% weight loss.

7. Myelosuppression and Neutropenia Studies.

Alteration in peripheral blood cell levels as a consequence of topotecanadministration was assessed over 4-6 weeks in ICR mice. Blood wascollected into EDTA microtainer tubes at Day 1, 3, 5, 7, 14, and 21following i.v. administration of free or liposome encapsulated topotecanat 10 mg/kg. Empty vesicles were administered as a control. CBC anddifferential analysis was performed at Central Labs for Veterinarians(Langley, BC) to quantify cellular levels, ratios and morphology.

8. Tumor Models.

The L1210 murine leukemia model and the CT-26 murine colon metastasesmodel were employed as in standard protocols. Human MX-1 and LX-1 celllines were obtained from the DCTD Tumor Repository in Frederick, Md.These cell lines were received as tumor fragments and were propagated inNCr nude mice by serial transplantation of 3×3 mm fragments. Experimentswere not initiated until the cell lines had been through 3 passages innude mice and the tumor lines were restarted when the passage numberreached 10.

9. Efficacy Studies.

All dosing of free and liposomal topotecan was administered by theintravenous route at 10 mVkg via the lateral tail vein. In the L1210 andCT-26 models, dosing occurred on day 1 (tumor cell injection=day 0). Forthe MX-1 and LX-1 tumor models, tumor volume was determined by repeatedperpendicular measurements of tumor dimensions and using the formula:

Volume (mm³)=(L×W²)/2

Dosing was initiated in the MX-1 and LX-1 models when tumors had clearlydemonstrated growth and were in the range 100-300 mm³.

Since most drugs exhibit a balance between a biological effect andtoxicity, it is useful to examine a parameter that incorporates both ofthese attributes. The most commonly employed parameter is therapeuticindex (TI). Traditionally, therapeutic index is defined as:

TI=LD₅₀/ED₅₀

However, since it is no longer permissible to perform LD50 studies,therapeutic index for these studies has been defined as follows:

TI=MTD/MED.

In the above formula, MTD is the maximum tolerated dose, defined as thatdose that causes a mean weight loss of 20% in a group of animals; andMED is the minimal effective dose, defined as the dose that produces anoptimal % T/C value of ≦40 in the solid tumor models or an % ILS of50±10% in the survival models.

B. Results

1. Pharmacokinetics and Drug Leakage.

The influence of liposome encapsulation and drug-to-lipid ratio onplasma pharmacokinetics and drug leakage of topotecan was examined over24 h in ICR mice. Liposome encapsulation of topotecan (drug-to-lipidratio, 0.11, wt/wt) had a dramatic influence on the pharmacokineticsparameters of the drug (see, FIG. 1, top; and Table 1). At a 5 mg/kgdose of topotecan, a 164-fold increase in plasma AUC, a 24-fold increasein C_(max) and a 24-fold increase in the plasma α half-life wereobserved for the liposomal drug relative to the free drug (see, Table1). Historically, large improvements in AUC and plasma half-lives ofliposomal drugs have resulted in enhanced delivery of the drug todisease-sites (such as tumors), a process known as “disease-sitetargeting”.

The formulations used in this study were prepared by the Mg-A23187ionophore method. There was an initial rapid release of drug in thefirst 10-30 minutes after iv administration (see, FIG. 1, bottom),followed by a more gradual release phase. The t_(1/2release) for theMn-A23187 and Mg-A23187 formulations were ˜3 h and ˜5-7 h, respectively;however, very little drug was present in either formulation at 24 h.

For most liposomal drug formulations, the pharmacokinetic properties ofthe encapsulated drug are controlled by the lipid composition and dose.

Liposomal topotecan has been shown to exhibit exceptional anti-tumoractivity, even at very low drug doses (0.5 mg/kg; drug-to-lipid ratio,0.10, wt/wt). At these drug doses and drug-to-lipid ratio, liposomeelimination from the plasma is expected to be rapid. Therefore, todetermine whether the pharmacokinetics of topotecan at low doses couldbe improved, a low drug-to-lipid ratio (0.02, wt/wt) formulation oftopotecan was investigated. Interestingly, in this study, the lowdrug-to-lipid ratio formulation released the drug much faster than thehigher drug-to-lipid ratio (0.11, wt/wt) formulation. This result wasunexpected.

TABLE 1 Pharmacokinetic parameters of free and liposomal topotecan. DoseAUC Cmax Cl α_(1/2) β_(1/2) Formulation (mg/kg) (h · μg/ml) (μg/ml)(ml/h) (h) (h) Free 1 1.97 0.75 13.9 0.14 11.8 5 2.77 2.17 49.6 0.2611.4 TCS 1 65.7 16.3 0.417 2.79 5 453 51.0 0.302 6.16All parameters were derived from one or two-compartment models usingWINNONLIN PK modeling software.

2. Maximum Tolerated Doses.

Single and multidose MTD studies were performed in tumor bearing Balb/c,BDF-1 and NCr nu/nu mice. Body weights of individual mice were monitoredthroughout each study to evaluate the general tolerability of free andliposomal topotecan and, where possible, to establish an MTD (see, FIG.2). The maximum tolerated dose of liposomal topotecan was 10 mg/kg on asingle administration, 7.5 mg/kg on a q7 d×3 schedule and 5 mg/kg on aq3 d×4 schedule. The reported LD₁₀ of free topotecan following a singleintravenous infusion in mice is 75 mg/M² (˜25 mg/kg) [Hycamtin™ productmonograph]; however, very little weight loss was observed at doses up to40 mg/kg, although this was considered the MTD due to acute responses.Drug quantities were limited so doses higher than 40 mg/kg (administeredover 5-10 minutes) were not pursued. It has previously been indicatedthat the LD₁₀ of free topotecan on a qd×5 schedule is 14 mg/M2/dose(˜4.7 mg/kg/dose) (Grochow, et al. Drug Metab. Dispos. 20:706-713(1992)).

3. Toxicity.

The major dose-limiting toxicity of free topotecan administered daily inhumans for 5 consecutive days (d×5) at 1.5 mg/M²/dose, the MTD, isnon-cumulative myelosuppression. As mentioned earlier, humans are moresensitive than mice to myelosuppression and can only tolerate 11% of theMTD in mice (1.5 vs 14 mg/M²). In this regard, dogs have been shown tobe a much better predictor of topotecan myelosuppression in humans(Burris, et al., J. Natl. Cancer Inst. 84:1816-1820 (1992)). However,mice should be suitable for comparing the relative myelosuppressiveeffects of free and Liposome encapsulated topotecan.

In a study, the maximal reduction in peripheral WBC counts occurred atday 3 post-injection following administration of liposomal topotecan. Acomparison of peripheral blood cell levels and morphology was then madeat day 3 following administration of free or liposome encapsulatedtopotecan or empty vesicles (see, Table 2). The dose used for thiscomparison was the MTD of liposome-encapsulated topotecan (10 mg/kg). Asignificant reduction in circulating neutrophils was observed forliposomal topotecan relative to free topotecan (˜40-fold), emptyvesicles (˜10-fold) or control animals (˜20-fold). Total WBC levels andthe lymphocyte sub-population were reduced approximately 2-fold forliposomal topotecan relative to control animals. No significantdifferences were observed in these parameters for free topotecan at thesame dose. At day 21 post-injection total, WBC levels for liposomaltopotecan remained approximately 2.5-fold lower than normal animals;however, neutrophils levels had recovered from a 20-fold decrease to a3-fold decrease relative to normal mice. Lymphocyte levels remained˜2-fold lower than normal mice. No other significant differences wereobserved.

Analysis of serum chemistry parameters at day 3 post-injection revealedvery few changes relative to untreated animals (see, Table 3). The onlychange of note was a statistically significant increase (˜2-fold) inglobulin levels and a concomitant decrease in the albumin/globulin ratiofor animals treated with liposomal topotecan. No other significantchanges were observed.

TABLE 2 Blood CBC and differential of ICR mice treated with a 10 mg/kgi.v. dose of free or liposome encapsulated topotecan. Day Post- WBCDifferential Treat- Injec- WBC Neutro Lympho Mono Eosino Baso RBC Hb HcPLT ment tion (×10⁹/L) (×10⁹/L) (×10⁹/L) (×10⁹/L) (×10⁹/L) (×10⁹/L)(×10¹²/L) (g/L) (L/L) (×10⁹/L) Control 6.47 ± 1.62 0.937 ± 0.201 5.23 ±1.45  0.180 ± .0.042 0.059 ± 0.039 0.056 ± 0.053 8.67 ± 0.93 142 ± 120.438 ± 0.045 717 ± 317 Free 3 6.70 ± 1.95 0.520 ± 0.200 5.90 ± 1.700.177 ± 0.072 0.031 ± 0.021 0.057 ± 0.040 8.47 ± 0.39 136 ± 05 0.444 ±0.012 879 ± 145 21 5.16 ± 1.18 0.480 ± 0.122 4.33 ± 0.93 0.247 ± 0.1800.034 ± .016  0.088 ± 0.071 9.81 ± 0.37 154 ± 04 0.493 ± 0.014 907 ± 059TCS 3 2.82 ± 1.05 0.048 ± 0.018 2.63 ± 0.87 0.109 ± 0.126 0.001 ± 0.0010.034 ± 0.029 8.93 ± 0.76 141 ± 10 0.463 ± 0.033 564 ± 098 21 2.54 ±1.43 0.282 ± 0.167 2.06 ± 1.16 0.131 ± 0.142 0.019 ± 0.011 0.064 ± 0.0609.41 ± 0.83 154 ± 12 0.486 ± 0.035 1009 ± 161  Empty 3 4.68 ± 1.13 0.598± 0.238 3.66 ± 0.93 0.248 ± 0.168 0.081 ± 0.044 0.064 ± 0.055 7.77 ±0.30 130 ± 05 0.416 ± 0.014 863 ± 143 21 5.05 ± 0.64 0.898 ± 0.575 3.78± 0.88 0.263 ± 0.163 0.038 ± 0.036 0.072 ± 0.057 9.36 ± 0.67 152 ± 080.483 ± 0.033 1366 ± 144 

TABLE 3 Serum chemistry panel of ICR mice treated with a 10 mg/kg i.v.dose of free or liposome encapsulated topotecan - day 3 post-injection.BUN Creatinine TP Albumin Globulin Alb/Glob Bilirubin Alk Phos ALT ASTCPK Treatment (mmol/L) (μmol/L) (g/L) (g/L) (g/L) Ratio (μmol/L) (IU/L)(IU/L) (IU/L) (IU/L) Control 11.3 ± 3.0 83 ± 6  46.7 ± 2.1  31.3 ± 1.515.3 ± 1.2  2.07 ± 0.15 4.7 ± 0.6 86 ± 12 27 ± 31 59 ± 22  87 ± 107 Free 9.4 ± 3.2 82 ± 18 48.0 ± 2.1  32.8 ± 1.3 15.2 ± 1.1  2.16 ± 0.15 3.8 ±0.8 67 ± 35 13 ± 23 55 ± 10 56 ± 38 TCS 10.0 ± 3.9 96 ± 28 55.8 ± 11.828.8 ± 2.5 27.0 ± 10.1 1.18 ± 0.33 2.5 ± 0.6 73 ± 21 23 ± 17 77 ± 29 155± 54  Empty ND 68 ± 13 49.3 ± 1.2  33.0 ± 1.7 16.3 ± 0.6  2.00 ± 0.174.3 ± 0.6 70 ± 10 17 ± 15 53 ± 6  56 ± 26

C. Efficacy Studies in Murine and Human Tumor Models: Single DoseStudies

1. L1210 Murine Leukemia.

The intravenous L1210 murine leukemia model has been used extensively toevaluate differential activity between free and liposome encapsulatedchemotherapeutic agents and was one of the original (1955-1975) modelsin the in vivo NCI screen of novel chemotherapeutic agents (Plowman, etal., Human tumor xenograft models in NCI drug development. In“Anticancer Drug Development Guide Preclinical Screening, ClinicalTrials, and Approval” (B. Teicher, Ed.), Humana Press Inc., Totowa(1997); Waud, Murine L1210 and P388 leukemias. In “Anticancer DrugDevelopment Guide: Preclinical Screening, Clinical Trials, and Approval”(B. Teicher, Ed), Humana Press Inc., Totowa (1997)). The model israpid—the mean survival of untreated animals is typically ˜7-8 days—andthe administered tumor cells seed in the liver and bone marrow.

Administration of free topotecan as a single intravenous dose hadminimal effect on survival in the L1210 model (see, FIG. 3A). At thehighest dose of free topotecan, a median survival of 13 days (44% ILS)was observed. There was one long-term survivor (day 60) in this group.In contrast, a single i.v. administration of liposomal topotecan ateither 5 or 10 mg/kg resulted in 100% survival at day 60 (see, FIG. 3B).Median survival for a 1 mg/kg dose was 13 days (44% ILS) and thesurvival curve was nearly identical to that of the free topotecanadministered at 30 mg/kg—a 30-fold improvement in potency. At higherdoses (30 mg/kg) of the liposomal topotecan, toxic deaths were observed.The MTD for liposomal topotecan was 20 mg/kg in BDF-1 mice after asingle i.v. administration.

2. CT-26 Murine Colon Carcinoma.

The murine CT-26 colon cell line is useful for drug screening since itreadily grows as subcutaneous solid tumors or can be administeredintravenously and used as a survival model. In addition, when the tumorcells are administered by intrasplenic injection, followed bysplenectomy, the cells seed to the liver and give rise to anexperimental metastases model that more closely resembles the clinicalprogression of colorectal cancer. The model has been used extensivelyand is described, for example, in detail elsewhere.

In the CT-26 model, administration of a single dose of topotecan had amodest impact on survival resulting in % ILS of 23-60% over the doserange 5-40 mg/kg (see, FIG. 4). Liposome encapsulated topotecan,however, was highly active at doses greater than 5 mg/kg, resulting in100% survival (8/8) at day 90. At 10 mg/kg, 87.5% survival (7/8) wasobserved at day 90; however, the tumor burden in dead animal was verylow suggesting that this animal may have died due to other factors, suchas infection related to myelosuppression. A dose response was observedfor liposomal topotecan, with the 2 mg/kg dose giving an % ILS of 54%.This was determined to be the MED and was comparable to the % ILS (58%)achieved using free topotecan at 40 mg/kg—a 20-fold increase in potency.

3. MX-1 Human Breast Carcinoma.

MX-1 is an experimental model of human breast cancer and has a reporteddoubling time of 3.9 days (NCI); in this study, the median doubling timewas consistently 3.6-13.7 days. The tumor cell line was derived from theprimary tumor of a 29-year-old female with no previous history ofchemotherapy and is provided by the DCTD (NCI) tumor repository as atumor fragment that is serially passaged in nude mice. Histologically,MX-1 is a poorly differentiated mammary carcinoma with no evidence ofgland formation or mucin production. MX-1 was one of 3 xenograft models(MX-1, LX-1, CX-1) that comprised the NCI in vivo tumor panel andprescreen (0.1976-1986) for evaluating novel chemotherapeutic agents(Plowman, et al., Human tumor xenograft models in Na drug development.In “Anticancer Drug Development Guide: Preclinical Screening, ClinicalTrials, and Approval” (B. Teicher, Ed), Humana Press Inc., Totowa(1997)). Since then, MX-1 has been incorporated into a larger panel ofbreast tumor models (12 in total) to reflect a shift in NCI strategyfrom “compound-oriented” discovery to “disease-oriented” discovery.

In staged (100-300 mm³) MX-1 tumors, free topotecan exhibiteddose-dependent inhibition of tumor growth (see, FIG. 5; Table I). At thehighest dose (40 mg/kg), an optimal % T/C of 24% was obtained; whileoptimal % T/C values for 10 and 5 mg/kg were 66% and 78%, respectively.No drug-related deaths were observed and all animals gained weightthroughout the study. Liposome encapsulation of topotecan had a markedimpact on % T/C, with optimal % T/C values of 8%, -49% and -62%following a single administration of the drug at 2, 5 or 10 mg/kg,respectively. A negative % T/C value is indicative of tumor volumeregression from the original staged tumor size (100-300 mm³). Accordingto NCI guidelines, an optimal % T/C<10% is considered significantactivity, while values<42% are the minimum acceptable limits foradvancing a drug further in development (Corbett, T. et al., In vivomethods for screening and preclinical testing. In “Anticancer DrugDevelopment Guide: Preclinical Screening, Clinical Trials, and Approval”(B. Teicher, Ed.), Humana Press Inc., Totowa (1997)). Liposomeencapsulation increased the toxicity of topotecan, reducing the MTD to10 mg/kg from >40 mg/kg for free topotecan.

4. LX-1 Human Lung Carcinoma.

LX-1 is an experimental model of human small cell lung cancer (SCLC).The tumor cell line was derived from the surgical explant of ametastatic lesion found in a 48 year old male and is provided by theDCTD (NCI) tumor repository as a tumor fragment that is seriallypassaged in nude mice. The LX-1 model was part of the NCI in vivo tumorpanel from 1976-1986 (Plowman, J. et al., Human tumor xenograft modelsin NCI drug development. In “Anticancer Drug Development Guide:Preclinical Screening, Clinical Trials, and Approval” (B. Teicher, Ed.),Humana Press Inc., Totowa (1997)) and, although used less frequentlynow, remains a useful xenograft model for comparative activity studiesbetween free and liposomal drugs because of its rapid growth rate.

In general, the LX-1 model was less sensitive to the effects oftopotecan than the MX-1 model, for both free and liposome-encapsulateddrug (see, FIG. 6; Table I). Optimal % T/C values for free topotecanwere 43%, 55% and 67% for doses of 30, 10 or 5 mg/kg, respectively.Anti-tumor activity was improved through encapsulation, resulting in %T/C values of 8%, 11% and 13% for doses of 30, 10, or 5 mg/kg,respectively. Interestingly, all of the liposomal topotecan dosesexhibited similar activity. This was an early study and subsequentstudies in other models (see, FIGS. 4-6) indicate dose responsebeginning at doses<5 mg/kg. This is consistent with the observation thatcamptothecin-class compounds (and presumably other antineoplasticagents) can exhibit “self-limiting” efficacy whereby, at doses above acritical threshold dose, no further activity benefits are observed(Thompson, Biochim. Biophys. Acta 1400:301-319 (1998)). This situationcould conceivably occur if the drug has limited tumor cell access or ifthe drug is acting on, and destroying, the tumor vasculature (i.e., hasanti-angiogenic activity). In both instances, a higher dose of drugwould be expected to have negligible benefit.

As observed in the L1210 study, encapsulation of topotecan enhanced thetoxicity of the drug and reduced the MTD. The MTD in tumor-bearing nudemice was 10 mg/kg (˜16% weight loss). At 30 mg/kg, 4/6 drug-relatedtoxic deaths were observed and maximum weight loss reached ˜29% (27-34%range).

D. Efficacy Studies in Murine and Human Tumor Models: Multiple DoseStudies

1. MX-1 Human Breast Carcinoma.

To address the effectiveness of multiple administration and prolongedexposure of the tumors to drug, two multiple dose protocols wereexamined in MX-1 xenografts—q3 d×4 and q7 d×3 schedules. On the q4 d×3schedule, free topotecan exhibited moderate activity at 2.5 and 10mg/kg/dose and minimal activity at 1.25 mg/kg/dose (see, FIG. 7; TableII). Optimal T/C values for free topotecan on this dosing schedule were55%, 30% and 27% for 1.25, 2.5 and 10 mg/kg/dose, respectively. For theencapsulated topotecan administered on the same dosing schedule, optimal% TIC values were -15%, -100%, 100%, and -100% for 0.5, 1.25, 2.5 and 5mg/kg/dose, respectively. All regressed tumors were monitored for 60days. At the end of this period, all animals treated with 1.25mg/kg/dose of liposomal topotecan were considered tumor free.

On a q7 d×3 dosing schedule, little activity was observed with the freetopotecan, either a 5 or 10 mg/kg/dose (see, FIG. 8; Table II). At thesame doses, liposomal topotecan induced complete regression of thestaged tumors. However, on this dosing schedule, 10 mg/kg/dose was tootoxic and this portion of the study was halted as 6/6 toxic deaths (oreuthanasia's) were observed by day 24.

2. LX-1 Human Lung Carcinoma.

Initial studies (single dose) in the LX-1 model indicated that freetopotecan was inactive at evaluated doses<30 mg/kg and liposomaltopotecan inhibited tumor growth, but did not induce regression. Toimprove this activity, a multiple (q7 d×3) schedule was examined forboth free and liposomal topotecan. In this instance, considerablygreater activity was observed for free topotecan compared to the singledose study and optimal % T/C values of 5 and 40 were obtained for 30 and10 mg/kg/dose, respectively. Liposomal topotecan also exhibitedsignificantly improved activity, resulting in complete regression (withsubsequent re-growth) at 5 mg/kg/dose. Optimal % T/C values forliposomal topotecan in this model and dosing schedule were—55, 3 and 16for 5, 2.5, 1.25 mg/kg/day, respectively.

3. Therapeutic Index (TI) Comparisons.

The therapeutic index of free and liposomal topotecan was assessed in 4different tumor models on several different dosing schedules (see, Table4). The assumptions and definitions used to generate these numbers arefound in Table III. In some instances, a true MED or MTD was notobserved and was therefore estimated mathematically based on doseresponse trends. For instance, an acute MTD of 40 mg/kg was observed forfree topotecan administered as a single bolus injection, but the trueMTD (based on weight loss) would likely be closer to 60 mg/kg if thedrug was infused over 5-10 minutes. Also, complicating the analysissomewhat was the level of potency of the liposomal formulation.Significant anti-tumor activity was achieved at low drug doses and theMED had to be estimated in certain studies. In these instances, anotation was made in Table 4.

In general, the increase in therapeutic index for liposomal topotecanwas relatively large for single dose administration (5, 10, 15 and18-fold, depending on the model) and decreased with increasing dosingfrequency. This is illustrated in Table 4, where the TI_(TCS)/TI_(Free)ratio was 4.7-7.5 and 3.3 for q7 d×3 and q3 d×4 schedules, respectively.The decrease in the TI_(TCS)/TI_(Free) ratio with more frequent dosingis consistent with preclinical and clinical studies indicating that theefficacy and toxicity of free topotecan is schedule-dependent.

TABLE 4 Relative Therapeutic Indices of Free and Liposomal Topotecan inMurine and Human Tumor Models.^(a) Route of Dosing Tumor Inocu- Sched-Model lation ule TI_(Free) TI_(TCS) TI_(TCS)/TI_(Free) L1210 i.v. single1.3 (2.0)^(b) 20  15.4 (10)^(b) (murine leukemia) CT-26 i.s. single 1.0(1.5)^(b)  5.0 5 (3.3)^(b) (murine colon) MX-1 s.c. single 1.4 (2.1)^(b)25  17.9 (11.9)^(b) (human q3 d × 4 15   50^(c)  3.3 breast) q7 d × 32.0   15.0^(c) 7.5 LX-1 s.c. single 1.3 (2.0)^(b) 13.3 10.2 (6.7)^(b)(human q7 d × 3 4.0 18.8 4.7 lung) ^(a)based on data in Table II andIII; formulas and definitions in Table IV. ^(b)obtained using an acuteMTD of 40 mg/kg; second value is based on an estimated MTD (body weight)^(c)a conservative estimate that may be ~2-fold greater; difficult toassess the MED due to high activity at low doses.

E. Discussion

Topotecan is an excellent candidate for liposome encapsulation. Briefly,topotecan is cell-cycle specific (S-phase) and activity is greatlyenhanced with prolonged exposure, topotecan exhibits rapid plasmapharmacokinetics and the drug needs to be maintained below pH 6.0 toretain biological activity. This is an ideal scenario for using arelatively non-leaky liposome formulation (such as SM:CH, 55:45) thathas an acidic aqueous core. The required acidic interior can beproduced, for example, by pH-loading or ionophore loading methodology.Here, it has been demonstrated that encapsulation of topotecan in SM/CHLiposomes by the Mg-A23187 method results in dramatic enhancements inanti-tumor efficacy. Modest enhancement of toxicity was also observedfor liposomal topotecan, but this was largely offset by substantial dosereductions that achieved comparable and, in most instances, superiorefficacy relative to the free drug.

Therapeutic index (TI) is a useful parameter of drug activity, as it ismeasure of the ratio of toxicity (MTD) to biological activity (userdefined endpoint, i.e., MED, ED₅₀, or ED₈₀). In general, the lower theTI, the greater the risk of toxicity since the dose of drug required toelicit a biological effect approaches the MTD. Therapeutic index isparticularly useful for the evaluation of liposomal drugs since therelative change in TI can be used to define the benefit (or lackthereof) of encapsulation. As demonstrated herein, the TI improved from3-18 fold depending on the model and dose schedule used. Therefore, theimprovement in biological activity observed following liposomeencapsulation of topotecan more than compensates for any increases intoxicity.

Without intending to be bound by any theory, it is thought that thesignificant improvements in anti-tumor activity and the increasedtoxicity of the liposomal form of the drug result from improvedpharmacokinetics and the maintenance of the drug in the active lactoneform. In these studies, 84% of topotecan was present in plasma as thelactone species after 24 h compared to 48% lactone for free topotecanafter only 5 minutes. Moreover, when the same dose (10 mg/kg) of freeand liposomal topotecan was administered intravenously in mice, theconcentration of lactone was ˜40-fold higher at times<1 h. At 24 h, thelactone plasma concentration for liposomal drug was 5.4 μg/ml compared,to 1.5 μg/ml at 5 minutes for free drug—still 3.5-fold greater than thepeak lactone concentration for free topotecan.

TABLE I Summary of Single Dose Anti-Tumor Activity and ToxicityParameters Anti-Tumor Activity Toxicity Model Dose % T/C^(a) T-C^(b) %ILS^(c) LCK^(d) TF^(e) DRD^(f) MWL^(g) L1210 Free 5 11 0/8 0/8 + (i.v.)Free 10 22 0/8 0/8 + NCTEF-005 Free 20 33 0/8 0/8 + Free 30 44 0/8 0/8 +Free 40 55 0/8 0/8 + TCS 1 44 0/8 0/8 + TCS 5 ** 8/8 0/8 + TCS 10 ** 8/80/8 −9.7 TCS 20 ** 7/7 1/8 −14.8 TCS 30 ** 3/3 5/8 −23.4 CT-26 Free 5 310/8 0/8 + (i.a.) Free 10 23 0/8 0/8 + NCTEF-005 Free 40 58 1/8 0/8 −0.4TCS 2 54 0/8 0/8 + TCS 5 ** 8/8 0/8 −6.8 TCS 10 ** 7/8 0/8 −19.1 MX-1Free 5 78 0.2 0 0.02 0/6 0/6 + (s.c.) Free 10 66 1.4 13 0.12 0/6 0/6 +NCTEF-004 Free 40 24 4.2 35 0.35 0/6 0/6 + TCS 2 8 7.4 65 0.62 0/6 0/6 +TCS 5 −49 10.2 74 0.85 0/6 0/6 −0.4 TCS 10 −62 14.2 83 1.19 1/6 0/6−18.3 LX-1 Free 5 67 1.4 0 0.13 0/6 0/6 + (s.c.) Free 10 55 1.9 0 0.180/6 0/6 + NCTEF-003 Free 30 43 2.9 7 0.27 0/6 0/6 −1.3 TCS 5 13 7.9 300.74 0/6 0/6 −1.7 TCS 10 11 8.7 22 0.82 0/6 0/6 −15.6 TCS 30 8 9.9 220.93 0/6 4/6 −29.0 ^(a)optimal % T/C following final treatment. Negativevalue indicates tumor regression. ^(b)tumor growth delay (difference intime for treated and control tumors to reach 500 mm³). ^(c)increase inlifespan relative to untreated animals (expressed as %). ^(d)log cellkill (gross). ^(e)tumor free animals at the end of study (i.e. novisible tumors or long term survivors). ^(f)drug related deaths.^(g)maximum mean weight loss per treatment group. ^(h)positive weightchange (i.e. at no time did weight decrease below pre-treatment weight).** long term survivors

TABLE II Summary of Multiple Dose Anti-Tumor Activity and ToxicityParameters Anti-Tumor Activity Toxicity Model Dose % T/C^(a) T-C^(b) %ILS^(c) LCK^(d) TF^(e) DRD^(f) MWL^(g) MX-1 Free 1.25 55 2.0 20 0.17 0/60/6 +^(h) (q3dx4) Free 2.5 30 5.0 55 0.42 0/6 0/6 + NCTEF-006 Free 10 272.5 52 0.21 1/6 0/6 + TCS 0.5 −15 23.5 157 1.96 1.6 0/6 −0.3 TCS 1.25−100 ** ** 6/6 0/6 −1.0 TCS 2.5 −100 ** ** 6/6 0/6 −11.5 TCS 5 −100 **** 6/6 0/6 −20.0 MX-1 Free 5 58 1.8 27 0.15 0/6 0/6 + (q7dx3) Free 10 612.0 ND^(i) 0/6 0/6 −0.8 NCTEF-009 TCS 5 −100 ** ** 6/6 0/6 −7.6 TCS 10−100 ND^(i) ND^(i) 6/6 6/6 −29.0 LX-1 Free 10 40 2.0 21 0.14 0/6 0/6−6.2 (q7dx3) Free 30 5 20.9 58 1.53 0/6 0/6 −8.8 NCTEF-007 TCS 1.25 1610.8 54 0.79 0/6 0/6 −7.7 TCS 2.5 3 23.2 79 1.70 0/6 0/6 −7.3 TCS 5 −5530.2 100 2.22 0/6 0/6 −10.5 LX-1 Free 10 28 4.4 41 0/6 0/6 −3.6 (q7dx3)Free 30 9 25 72 0/6 2/6 −16.4 NCTEF-011 TCS 7.5 ND^(i) ND^(i) ND^(i) 0/66/6 >−30 TCS 0.75 27 11.2 50 0/6 0/6 −1.3 ^(a)optimal % T/C followingfinal treatment. Negative value indicates tumor regression. ^(b)tumorgrowth delay (difference in time for treated and control tumors to reach500 mm³). ^(c)increase in lifespan relative to untreated animals(expressed as %). ^(c)log cell kill (gross). ^(e)tumor free animals atthe end of study (i.e. no visible tumors or long term survivors).^(f)drug related deaths. ^(g)maximum mean weight loss per treatmentgroup. ^(h)positive weight change (i.e. at no time did weight decreasebelow pre-treatment weight). ^(i)not determined; toxic deaths in theliposome-encapsulated group. ** “cures”; no visible tumors by day 60.

TABLE III Definitions and Formulas for Toxicity and Anti-Tumor ActivityParameters DRD Drug-related death. A death was considered drug-relatedif the animal died or was euthanized within 15 days following the finaltreatment with drug AND its tumor weight was less than the lethal burdenon control mice, or its weight loss was greater than 20% that of thecontrol animals. GI₅₀ The concentration of drug that causes 50% growthinhibition in a population of cells in vitro. The NCI renamed the IC₅₀parameter to emphasize the correction for cell count at time zero.Therefore, the formula is: GI₅₀ = (T − T_(o))/(C − T_(o)) × 100 = 50 Tand T_(o) are the optical densities at 48 and 0 h, respectively; C isthe control (cell count) optical density at 0 h. % ILS Increase inlifespan (in percent). For survival models this is calculated using themedian survival times for the treated (T_(treat)) and control (T_(cont))animals, according to: (T_(treat) − T_(cont))/T_(cont) × 100 For thesolid tumor models, the time for tumors to reach 2000 mm³ (~10% of bodyweight) was used as an ethical cutoff instead of median survival. LCKLog cell kill (gross). This parameter estimates the number of log₁₀units of cells killed at the end of treatment, according to the formula:(T − C) × 0.301/median doubling time Net log cell kill can be calculatedby subtracting the duration of treatment from the tumor growth delay (T− C) parameter as follows: [(T − C) − duration of treatment] ×0.301/median doubling time A log cell kill of 0 indicates that the cellpopulation at the end of treatment is the same as it was at the onset oftreatment. However, a log cell kill of 4, for example, indicates a99.99% reduction in the initial cell population. MBWL Maximum bodyweight loss (in percent). The animals are weighed prior to the firstadministration of the drug (Wi) and on various days during the study(Wd). The percent change in body weight is calculated by: MBWL = (W_(d)− W_(i))/W_(i) × 100 MED Minimum effective dose. This is a somewhatarbitrary parameter. For these studies we have defined the MED as thelowest dose achieving an optimal % T/C ≦ 40 (for solid tumor models) ora % ILS of 40-60% (for survival models). MTD Maximum tolerated dose.Dose of drug that results in a MBWL of ≦20%. % T/C Optimal ratio oftreated vs control tumors obtained following the first course oftreatment. These values are obtained by subtracting the median tumorweight on the first day of treatments (T_(i) or C_(i)) from the tumorweights on each observation day according to the following formula: %T/C = (Δ T/Δ C) × 100, where Δ T > 0, or % T/C = (Δ T/T_(i)) × 100,where Δ T < 0 According to NCI activity criteria, the following scoringsystem applies (Plowman, et al., Human tumor xenograft models in NCIdrug development. In “Anticancer Drug Development Guide: PreclinicalScreening, Clinical Trials, and Approval” (B. Teicher, Ed.), HumanaPress Inc., Totowa (1997)[22]: 0 = inactive, % T/C > 40 1 = tumorinhibition, % T/C range 1-40 2 = tumor stasis, % T/C range 0 to −40 3 =tumor regression, % T/C range −50 to −100 4 = % T/C range −50 to −100and >30% tumor-free mice TGD Tumor growth delay (also represented as T −C). This parameter expresses the difference in time (in days) fortreated and control tumors to attain an arbitrary size (typically 500 or1000 mm³). TI Therapeutic index. Therapeutic index is the ratio of atoxicity parameter (i.e. LD₅₀, LD₁₀, MTD) and a biological activityparameter (i.e. ED₅₀—the dose that causes a defined biological responsein 50% of the treatment group). In general, TI describes the margin ofsafety for a drug. For animal model studies this is traditionallydescribed by the formula: TI = LD₅₀/ED₅₀ However, since it is no longerethically permissible to perform LD₅₀ studies, we have definedtherapeutic index for these studies as: TI = MTD/MED

It is to be understood that the above description is intended to beillustrative and not restrictive. Many embodiments will be apparent tothose of skill in the art upon reading the above description. The scopeof the invention should, therefore, be determined not with reference tothe above description, but should instead be determined with referenceto the appended claims, along with the full scope of equivalents towhich such claims are entitled. The disclosures of all articles andreferences, including patent applications and publications, areincorporated herein by reference for all purposes.

1. A liposomal topotecan unit dosage form, said unit dosage formcomprising: a lipid; and a topotecan dosage of from about 0.01mg/M²/dose to about 7.5 mg/M²/dose, wherein said liposomal topotecanunit dosage form has a drug:lipid ratio (by weight) of about 0.05 toabout 0.2.
 2. The liposomal topotecan unit dosage form of claim 1,wherein said drug:lipid ratio (by weight) is about 0.05 to about 0.15.3. The liposomal topotecan unit dosage form of claim 1, wherein saidlipid comprises a mixture of sphingomyelin and cholesterol.
 4. Theliposomal topotecan unit dosage form of claim 1, wherein said lipidcomprises sphingomyelin and cholesterol in a ratio by weight of about30:70 to about 60:40.
 5. The liposomal topotecan unit dosage form ofclaim 1, comprising from about 1 mg/M²/dose to about 4 mg/M²/dose oftopotecan.
 6. A liposomal topotecan formulation, wherein said liposomaltopotecan formulation retains greater than 50% active lactone speciesafter 12 hours in blood circulation.
 7. The liposomal topotecanformulation of claim 6, wherein said liposomal topotecan formulationretains greater than 80% active lactone species after 12 hours in bloodcirculation.
 8. A liposomal topotecan formulation comprising topotecan,sphingomyelin, cholesterol and a divalent cation ionophore.
 9. Theliposomal topotecan formulation of claim 8, wherein said divalentionophore is present in trace amounts.
 10. The liposomal topotecanformulation of claim 8, comprising a drug:lipid ratio (by weight) ofabout 0.05 to about 0.2.
 11. The liposomal topotecan formulation ofclaim 10, wherein said drug:lipid ratio (by weight) is about 0.05 toabout 0.15
 12. The liposomal topotecan formulation of claim 11,comprising trace amounts or greater of a divalent ionophore.
 13. Amethod of treating a solid tumor in a human afflicted therewith, saidmethod comprising administering to said human an effective amount of atopotecan dosage of claim 1 in a pharmaceutically acceptable carrier.14. The method of claim 13, wherein said solid tumor is selected fromthe group consisting of solid tumors of the lung, mammary, colon andprostate.
 15. The method of claim 13, further comprisingco-administration of a treatment for neutropenia or platelet deficiency.16. A method of treating solid tumors in a mammal, said methodcomprising: administering to said mammal having a solid tumor of thelung, mammary and/or colon a liposomal topotecan formulation having adrug:lipid ratio (by weight) of about 0.05 to about 0.2.
 17. A method oftreating solid tumors in a mammal, said method comprising: administeringto said mammal having a solid tumor of the lung, mammary and/or colon aliposomal topotecan formulation comprising from about 0.01 mg/M²/dose toabout 7.5 mg/M²/dose of topotecan for an interval regime, wherein saidinterval regime is once a day for at least two consecutive days.
 18. Themethod of treating solid tumors of claim 17, wherein said intervalregime is at least once a week.
 19. The method of treating solid tumorsof claim 17, wherein said interval regime is at least once every twoweeks.
 20. The method of treating solid tumors of claim 17, wherein saidinterval regime is at least once every three weeks.
 21. The method oftreating solid tumors of claim 17, wherein said liposomal topotecanformulation has a drug:lipid ratio (by weight) of about 0.05 to about0.2.
 22. A method of treating solid tumors in a mammal comprisingadministering to an animal having a solid tumor of the lung, mammaryand/or colon a liposomal topotecan formulation comprising from about0.01 to about 7.5 mg/M²/dose of topotecan every three days.
 23. Aliposomal camptothecin unit dosage form, said unit dosage formcomprising a lipid, a camptothecin dosage of from about 0.015 mg/M²/doseto about 1 mg/M²/dose and having a drug:lipid ratio (by weight) of about0.05 to about 0.2.
 24. The use of topotecan in the manufacture of amedicament comprising a liposome having a sphingomyelin to cholesterolratio (by weight) of from about 30:70 to about 60:40 for use in treatingsolid tumors in a mammal.
 25. The use of claim 24, for treating solidtumors of the lung, mammary and colon.